1. Field of the Invention
This invention relates to a method of measuring the decentering of a lens which is suitable for measuring the decentering amount of a curved surface, for example, the decentering amount of a spherical surface or an aspherical surface.
2. Related Background Art
In recent years, in various optical systems for use in digital cameras, video cameras, etc., aspherical lenses are often adopted for an improvement in optical performance and the downsizing of the optical systems.
Along with this, a method of accurately measuring the decentering amount of the optical axis (aspherical surface axis) of the aspherical lens is required.
As a method of measuring the decentering amount of the aspherical surface axis of the aspherical lens, there is known a method of measuring the profile data of an aspherical lens by the use of a three-dimensional position measuring apparatus (Japanese Patent Application Laid-open No. H07-229811).
According to the method disclosed in Japanese Patent Application Laid-open No. H07-229811, the decentering of an aspherical surface axis with the position of a lens holding member as the reference can be highly accurately measured without the lens holding member being rotated. It is also possible to measure the decentering of the aspherical surface axis relative to the outer diameter of a lens by three-dimensionally measuring the outer diameter portion of the lens. This method, however, requires a complicated three-dimensional position measuring apparatus. Also, as compared with two-dimensional profile measurement, the method includes a disadvantage that much time is required for measurement and data processing because enormous data is used.
At present, as a typical aspherical surface profile measuring apparatus, there is known Form Talysurf produced by TAYLOR-HOBSON LIMITED. Form Talysurf announced in 1984 is a two-dimensional profile measuring apparatus given the function of measuring a surface profile to a contact type surface roughness measuring apparatus.
The two-dimensional aspherical surface profile measuring apparatus calculates and evaluates the error amount between the cross-sectional shape of the design reference aspherical surface of a subject lens to be measured and the actually measured cross-sectional shape of the aspherical surface.
If the coordinate axis of the optical axis of the design reference aspherical surface profile and the coordinate axis of the optical axis of the actually measured aspherical surface profile coincide with each other, a simple difference is an aspherical surface shape error.
In an ordinary case, however, due to the existence of the decentering component of the subject lens, the positional error, the inclination error, or the like when the subject lens is held on the measuring apparatus, the coordinate axis of the optical axis of the actually measured aspherical surface profile does not coincide with the coordinate axis of the optical axis of the design reference aspherical surface shape.
So, generally, the aspherical surface profile measuring apparatus converts the coordinates on the basis of aspherical surface axis data searched for (calculated) from actually measured aspherical surface data so that the optical axis of the actually measured aspherical surface may coincide with the optical axis of the design reference aspherical surface, and calculates the error amount of the aspherical surface.
Heretofore, the searched for data of the position and inclination of the aspherical surface axis in a measurement scanning direction has been calculated from the actually measured profile of the subject lens. However, the decentering amount of the aspherical surface axis relative to the outer diameter of the subject lens could not be detected.
To measure the decentering amount of the aspherical surface axis relative to the outer diameter of the lens, the outer diameter portion of the lens and the profile of an aspherical surface lens should be measured at a time. However, in the conventional two-dimensional profile measuring method, it has been difficult to measure the outer diameter position of the lens simultaneously with the measurement of the aspherical surface profile.